Method and arrangement for washing and cooling at the outlet end of a continuous cellulose digester



Jan; 17; 1967 A Q A. LAAKSQ I 3,298,899 METHOD AND ARRANGEMENT FOR WASHING AND COOLING AT THE OUTLET END OF A CONTINUOUS QELLULOSE DIGESTE-R Filed Feb. 28, 1963 United States 3,298,899 Patented Jan. 17, 1967 3,298,899 i I METHOD AND ARRANGEMENT FOR WASHING AND COOLING AT THE OUTLET END OF A- CONTINUOUS CELLULOSE DIGESTER Oliver A. Laakso, Glens Falls, N.Y assignor to Aktiebolaget Kamyr, Karlstad, Sweden Filed FebgZS, 1963, Ser. No. 261,730 Claims priority, application Sweden, Mar. 3, 1962,

' 2,357/62 I 6 Claims. (Cl. 16219) The invention relates to a .method'and apparatus for performing, in an elongated cellulose digester wherein a fibrous material during its continuous feed in the longitudinal direction of the dig'esteris digested by cooking with a digesting liquorfat'high pressure andhi'gh temperature, a washing and cooling operation of the fibrous material immediately before the same vis discharged throgh an outlet at one end of the digester, usually in connection with relieving of at least the greater part ofthe digesting pressurej j For this purpose it 'is known to supply close to the digester outlet at liquid, such as for instance, Water or filtrate containing digesting liquor residues, which liquid is brought'to flow in CO lntepcurrent to the fibrous material, thereby cooling the fibrous material as well as washing digesting liquor out of the same.- Then the liquid supplied'at'the digester'outlet generally is of. a comparatively low temperature in order to cause an effective cooling of the fibrous material, and therefore the washing operation will also take place at alowtemperature.

However, in order to bring about an efiicient washing a high temperature is' required, because it is important not only to displace the digesting liquor and products dissolved thereby out of the spaces between the fibre bundles, but also to remove the chemicals from the finer pores of the material which can only be obtained by diffusion and therefore is favoured by'a high temperature.

The object-of the inventionis to provide an eificient cooling as well as an efiicient washing of the fibre material at the outlet end of the digester. In order to attain said object the-re is performed according to the invention 'awa'shing operation at a distance from the digester outlet bymeans of liquid which, exteriorly of the digester, is heated to a higher-temperature-than the temperature of the liquid supplied to the digester closer to its outlet for cooling purposes. Preferably, oneyand the same liquid quantity is used first for cooling and then after being heated, for washing of the fibrous material, and accordingto a secondary feature of the invention said object is realized whereby that liquid screened off the fibrous material and containing such liquid as consists of, or has been displaced by, the liquid supplied closer to the digester outlet forfcooling purposes, is heated exteriorly of the digester and is returned to the same as washing liquid. The liquid taken out of the digester in-order to be returned thereto afterbeing heated, can also contain liquid which already has passed one or more times through the fibrous fmaterial'inorder to wash the same, and thus a loop is formed in which'the liquid circulates, so that the same liquid quantity is brought to pass through the fibrous material repeatedly in one and the same path andbetween each such passage is heated in the part of the circulation loop located exteriorly of the digester. Preferably, the heating of the liquid supplied'or returned to the digester for washing purposes isperformed by exchanging heat with spent digesting liquor'takenout of the digester. I I

The invention will be more closely described hereinelow with reference to the accompanying drawing which diagrammatically shows an illustrative embodiment of a continuous cooking plant to which the invention is applied.

In the drawing, 11 designates a steaming vessel in which wood chipsor other finely comminuted cellulosic fibre material supplied through a low pressure rotary valve 15 is heated by low pressure steam supplied through the conduit 17 while the material is agitated and fed towards the outlet end of the steaming vessel by means of a feeding screw 19. The desired overpressure, approximately 1 atmosphere gauge, is maintained by means of an automatic control valve 21. Suitable digesting liquor, such as sodium lye, is supplied through the tube connection 22. The mixture of lye and chips is introduced into the upper end of an upright cylindrical digester by means of a high pressure rotary valve 23, and the mixture is I heated by steam which is'supplied from a conduit 26,

" girdles 35, 37-inserted in the digester wall, a pump 39, a

heat exchanger 41 and aret-urn conduit 43. This conduit, which is arranged centrally in the digester and has its lower end located at the centre of the digester cross-section and preferably somewhat above the level of the sieve girdle 35, reintroduces into the digester the circulating lye drawn off through the sieve girdles and heated in the heat exchanger. The circulating lye is heated to a temperature of approximately 170 C., and in the digester there is maintained a hydraulic pressure of the order of 10 atmospheres. During the slow and even descent of the fibre material through the digester it reacts with the digesting liquor, so that the fibrous material is completely digested when it reaches the level of the sieve girdle 53, inserted into the digester wall and located at a considerable distance from the outlet end of the digester, for instance at a distance from the lower endof the digester equal approximately to a third of the length of the digester. In the remaining bottom part of the digester the fibrous material is washed and cooled in a manner to.

be more completely described below, so that the fibrous material is almost completely relieved of digesting liquor and dissolved chemicals and has a considerably reduced temperature when it reaches the extreme lower end of the digester. There, the material is fed down into a centrally located outlet 45 by means of a motor-driven scraping or agitating device 47 and is transferred through a conduit 49 to a blow tank 44 in which atmospheric pressure or a low overpressure is maintained. Due to the pressure drop a certain steam quantity is given 01f, particularly if i in spite of the cooling at the digester outlet the material is still of a temperature above C. Said steam is discharged through an outlet 46 at the top of the blow tank, whereas the pulp is discharged through a bottom outlet 48 and is carriedthrough a conduit which is connected thereto and into which is inserted a valve 50, controlled by the level of pulp in the blow tank, to the pulp vat 52 of a rotary suction filter. Taken up on the rotary sieve drum 54.01: said filter is a pulp layer 56 which is. washed by washing water, e.g. warm or cold fresh water supplied by the spray nozzles 58. A part of the filtrate consisting of wash water and digesting liquor residues is collected in a container 60 from where it is returned by means of a pump 65 through the conduit 51 to the digester andis used therein in order to effect in accord- ;ance with the invention and in a manner to'be more closely described hereinbelow, a cooling operation at the outlet and a first washing of digester liquor out of the pulp already in the bottom part of the digester.

The sieve girdle53, consisting of a perforated or slitted I cylindrical sieve plate, is flush with the cylindrical wall of the digester, and outside of the sieve plate the digester wall is widened so that a collecting space 55 is formed for spent digesting liquor screened oi the fibre material moving past the sieve. This collecting space is connected by means of a conduit 57 to a steam separating device 59 preferably designed as a cyclone. The steam outlet located at the top thereof is connected by the abovementioned conduit 17 to the steaming vessel 11, and therefore a comparatively low overpressure is maintained in the cyclone. When the digesting liquor sieved-off enters the cyclone, it gives off a great part of its heat content as steam which is used for heating the chips in the steaming vessel. The digesting liquor is discharged at a low temperature through a bottom outlet and a conduit 72 in which is inserted a valve 73 for controlling the level of the liquid of the cyclone 59, into a second steam separating device 74 which preferably likewise is designed as a cyclone with a tangetial inlet of the conduit 72. In the cyclone 74 there is maintained a still lower overpressure than in the cyclone 59, or possibly atmospheric pressure. The steam given off therein can be used for heating purposes merely requiring lower temperature, such as for making warm water or for heating of the digesting liquor supplied through the tube connection 22. The digesting liquor discharged through the bottom outlet of the cyclone 74 is carried through a conduit 82 in which is inserted a valve 80 for controlling the liquid level of the cyclone, further on to a plant for recovery of the content of chemicals of the digesting liquor.

The conduit 51 is connected to a bore in the vertical driving shaft 87 of the scraping or agitating device 47, which bore communicates with passages having ejection orifices 89 distributed over the hub of the device and its radially extending arms, the liquid pumped in by the high pressure pump 65 being distributed approximately evenly across the crosssection of the digester. A part of said liquid passes together with the chips-liquid mixture out through the outlet 45 and serves to dilute the same to a consistency suitably low for its easy flow in the conduit 49. The remainder thereof, which serves for cooling and washing of the fibre material, is forced upwards in counter-current to the direction of motion of the fibre material and displaces the spent digesting liquor accompanying the same, so that the spent liquor is driven out through the sieve girdle 53 and is dispelled from the digester. Thus, the washing liquid replaces the digesting liquor in the spaces between the chip particles of the continuously descending chip column and moves upwardly through the same and then departs through the sieve girdle 53 together with digesting liquor.

According to the invention the digesting plant as above described is constructed so as to enable Washing in the lower part of the digester at a higher temperature than the temperature that the filtrate flowing through the conduit 51 naturally takes and which latter temperature may be suitable for cooling of the fibrous material in the outlet 45.

Inserted into the digester wall somewhat above the lower end of the digester is a further sieve girdle 86 which likewise consists of a cylindrical slitted or perforated sieve plate arranged flush with the digester wall part immediately above the same. Back of this sieve girdle in a still more widened part of the digester wall there is a collecting space 88 for liquid screened off. This space is connected to an exterior circulation conduit 90 into which are inserted a pump 92 and a heat exchanger 94. The conduit 90 continues in an interior conduit 96 extending along the axis of the digester end concentrically to the conduit 43 and the conduit 90 has an orifice at the center of the cross-section of the digester approximately at the level of the sieve girdle 86. One side of the heat exchanger 94 is inserted into the conduit 57, so that heat is exchanged between the hot digesting liquor passing through conduit 57 and the washing liquid passing through the conduit 90. Cooling liquid which has been supplied to the digester through the ejection holes 89 on the scraping or agitating device 47 and which penetrates upwardly while cooling the fibre material, departs through the sieve girdle 86 and is caused by the pump 92 to flow through the heat exchanger 94 wherein it is heated to a temperature considerably above C., for instance C., whereupon the liquid is returned to the digester via the tube 96, wherein it is spread from the orifice of the tube 96 radially in all directions. In the described circulation loop the liquid is caused to recirculate repeatedly before it is driven upwardly through the chip column towards the sieve girdle 53. In this manner there is effected at the level of the sieve girdle86 an efficient and thorough heating of the fibre material to the high temperature mentioned, so that the washing operation of the fibre material in the bottom end of the digester, on account of the diffusion of the chemicals from the interior of the fibre bundles out into the surrounding liquid being facilitated by the raised temperature, takes place considerably quicker and more efliciently than if the washing liquid supplied through the holes 89 had been driven directly upwards towards the sieve girdle 53.

Immediately below the sieve girdle 53 there is arranged a similar sieve girdle 98. Liquid screened off through the same is driven by a pump 100 through an exterior circulation conduit 102 and an interior conduit 104 connected thereto and extending along the axis of the digester, back to the digester and is spread from the orifice of said last-mentioned conduit radially in all directions approximately at the level of the sieves 53, 98. This transverse circulation of washing liquid through the chip column serves to equalize the concentration of digesting liquor over the digester cross-section, so that the least possible amount of digesting liquor remains in the coarser spaces of the material when the same enters the zone between the sieve girdles 98 and 86 where difiusion washing takes place. During its motion upwardly through the chip column in this part of the digester, the washing liquid is enriched with the chemicals extracted by diffusion. As already mentioned the washing liquid finally departs through the sieve girdle 53 together with the digesting liquor accompanying the chips and being displaced by the washing liquid.

The above-described embodiment is illustrative of a preferred form of the invention but is not intended to limit the possibility of insuring the features of improved washing and cooling at the outlet end of the digester. The arrangement disclosed herein is an example of apparatus in which the inventive features of the disclosure may be utilized, and it will become apparent to one skilled in the art that certain modifications may be made within the spirit of the invention as defined by the appended claims.

What is claimed is:

1. In a method of liberating cellulosic fibrous mate rial by digestion thereof with a digesting liquor at a high pressure and a high temperature in a continuous digester having a fibrous material inlet, a digesting liquid inlet, and a fibrous material outlet, the improvement of washing the fibrous material counter-current to the flow thereof and immediately before the same is discharged through the digester outlet, comprising supplying cooling liquid to the digester close to the digester outlet, screening off the liquid in the fibrous material and cooling liquid, heating said screened oif liquids exteriorly of the digester, returning the heated liquid as a washing liquid to the digester at a greater distance from the digester outlet than the cooling liquid is supplied and heating said washing liquid by exchanging heat with digesting liquid that is discharged from the digester prior to introducing the washing liquid.

2. In an apparatus for liberating cellulosic fibrous material by digestion thereof in a longitudinal cellulose digester with a digesting liquor at a high temperature and a high pressure and adapted such that the fibrous material will be continuously fed in the longitudinal direction of the digester, said apparatus having a fibrous material inlet, a digesting liquor inlet and an outlet for digested fibrous material, and said apparatus including means for heating said digesting liquor, the improvement which comprises providing said apparatus with a cooling liquid inlet positioned close to said outlet, a sieve girdle in said digester for screening off spent digesting liquor from the fibrous material, a sieve girdle between said cooling liquid inlet and said spent digesting liquor sieve girdle for screening ofi said cooling liquid, a second heating device exterior of the digester, a conduit connected to said cooling liquid sieve girdle, said conduit passing through said second heating device and returning to the digester, an orifice in said conduit within said digester between said cooling liquid inlet and said spent digesting liquor sieve girdle, said apparatus being adapted such that cooling liquid supplied through said cooling liquid inlet first cools the fibrous material and then washes it at a raised temperature.

3. An apparatus as set .forth in claim 2 in which said second heating device comprises a heat exchanger connected to said spent digesting liquid sieve girdle and to said cooling liquid sieve girdle for heating said cooling liquid with heat exchanged from the spent digesting fluid.

4. An apparatus as set forth in claim 2 further comprising a sieve girdle for screening off used washing liquid and means for recirculating said used washing liquid to the digester for further use in washing the fibrous material.

5. An apparatus-as set forth in claim 4 further comprising a sieve girdle disposed in said digester at a location upstream of said used washing liquid sieve girdle for screening off and finally discharging used washing liquid from the digester.

6. An apparatus as set forth in claim 4 further comprising means for driving said recirculating washing liquid radially outward from a central orifice of a conduit disposed within the digester and upstream of said used washing liquid sieve girdle,

References Cited by the Examiner UNITED STATES PATENTS 1,548,477 8/1925 Morterud 162-249 1,633,734 6/1927 Fish 162-249 2,695,232 11/1954 Richter 162-237 2,876,098 3/ 1959 Schandroch 162-60 3,007,839 11/1961 Richter 16217 3,035,963 5/1962 Schnyder 162--19 DONALL H. SYLVESTER, Primary Examiner.

H. CAINE, Assistant Examiner. 

1. IN A METHOD OF LIBERATING CELLULOSIC FIBROUS MATERIAL BY DIGESTION THEREOF WITH A DIGESTING LIQUOR AT A HIGH PRESSURE AND A HIGH TEMPERATUE IN A CONTINUOUS DIGESTER HAVING A FIBROUS MATERIAL OUTLET, THE IMPROVEMENT OF WASHING THE FIBROUS MATERIAL COUNTER-CURRENT TO THE FLOW THEREOF AND IMMEDIATELY BEFORE THE SAME IS DISCHARGED THROUGH THE DIGESTER OUTLET, COMPRISING SUPPLYING COOLING LIQUID TO THE DIGESTER CLOSE TO THE DIGESTER OUTLET, SCREENING OFF THE LIQUID IN THE FIBROUS MATERIAL AND COOLING LIQUID, HEATING SAID SCREENED OFF LIQUIDS EXTERIORLY OF THE DIGESTER, RETURNING THE HEATED LIQUID AS A WASHING LIQUID TO THE DIGESTER AT A GREATER DISTANCE FROM THE DIGESTER OUTLET THAN THE COOLING LIQUID IS SUPPLIED AND HEATING SAID WASHING LIQUID BY EXCHANGING HEAT WITH DIGESTING LIQUID THAT IS DISCHARGED FROM THE DIGESTER PRIOR TO INTRODUCING THE WASHING LIQUID.
 2. IN AN APPARATUS FOR LIBERATING CELLULOSIC FIBROUS MATERIAL BY DIGESTION THEREOF IN A LONGITUDINAL CELLULOSE DIGESTER WITH A DIGESTING LIQUOR AT A HIGH TEMPERATURE AND A HIGH PRESSURE AND ADAPTED SUCH THAT THE FIBROUS MATERIAL WILL BE CONTINUOUSLY FED IN THE LONGITUDINAL DIRECTION OF THE DIGESTER, SAID APPARATUS HAVING A FIBROUS MATERIAL INLET, A DIGESTING LIQUOR INLET AND AN OUTLET FOR DIGESTED FIBROUS MATERIAL, AND SAID APPARATUS INCLUDING MEANS FOR HEATING SAID DIGESTING LIQUOR, THE IMPROVEMENT WHICH COMPRISES PROVIDIDNG SAID APPARATUS WITH A COOLING LIQUID INLET POSITIONED CLOSE TO SAID OUTLET, A SIEVE GIRDLE IN SAID DIGESTER FOR SCREENING OFF SPENT DIGESTING LIQUOR FROM THE FIBROUS MATERIAL, A SIEVE GIRDLE BETWEEN SAID COOLING LIQUID INLET AND SAID SPENT DIGESTING LIQUOR SIEVE GIRDLE FOR SCREENING OFF SAID COOLING LIQUID, A SECOND HEATING DEVICE EXTERIOR OF THE DIGESTER, A CONDUIT CONNECTED TO SAID COOLING LIQUID SIEVE GIRDLE, SAID CONDUIT PASSING THROUGH SAID SECOND HEATING DEVICE AND RETURNING TO THE DIGESTER, AN ORIFICE IN SAID CONDUIT WITHIN SAID DIGESTER BETWEEN SAID COOLING LIQUID INLET AND SAID SPENT DIGESTING LIQUOR SIEVE GIRDLE, SAID APPARATUS BEING ADAPTED SUCH THAT COOLING LIQUID SUPPLIED THROUGH SAID COOLING LIQUID INLET FIRST COOLS THE FIBROUS MATERIAL AND THEN WASHES IT AT A RAISED TEMPERATURE. 